The invention relates to a reactive sound attenuator consisting of a cavity with sound-proof limits and including at least one membrane, an acoustic sensor in the immediate vicinity in or on said membrane, as well as an electroacoustic transducer and an inverting signal amplifier.
With reference to FIG. 5, so-called anti-noise systems shown therein are based on a simple concept. (Nelson, P. A., Elliott, S. J.: Active Control of Sound and Vibration. Academic Press Limited, London: 1992). These systems are most frequently aimed at and refined in active noise control in order to attenuate noise in ducts and passages. Here, an incident primary sound wave is detected by a microphone which is located in a duct and distinctly offset in front of the remaining components in a direction that is toward the noise source. The detected microphone signal is arithmetically rotated through 180xc2x0 as precisely as possible by means of a signal processor 11. This rotated signal serves to control a loudspeaker 9 which subsequently emits a secondary sound wave.
In an ideal case, both waves are superimposed on one another along the direction of the sound wave""s propagation until the wave is cancelled. This cancellation can be monitored by means of a second microphone 10 in the direction of the sound wave""s propagation. This second microphone 10 outputs a signal which, at the same time, may be used to adapt the signal processor to possible variations of sound propagation in the respective duct.
This procedure can be successfully performed very precisely by means of advanced signal processors at least under laboratory conditions. However, their practical application is characterised by a high responsiveness and sensitivity to superimposed air flows or temperature variations, as well as by a high cost for electronic elements and signal processing means.
German patent document DE 40 27 511 discloses a hybrid sound attenuator as shown in FIG. 6. This system is used to realize an optimum acoustic impedance of a duct wall 1 located on the front side of a known passive sub-system 12 via a supplementing active sub-system on the rear side. The acoustic characteristics of the passive sub-system form the starting point, e.g., a layer of porous absorber material. Further elements of this hybrid sound attenuator serve to generate a rear-side terminating impedance of the passive sub-system. The acoustic pressure behind the passive sub-system must be measured with a microphone 13 to enforce this terminating impedance. The microphone voltage is then fed back to a loudspeaker 14 via signal-shaping transducer 15. Here, the calculated impedance is expected to occur on the membrane surface of the loudspeaker.
This method requires that the signal-shaping transducer proposed in German patent document DE 40 27 511 must first compensate the intrinsic characteristics of all the electromechanical components (i.e., microphone, loudspeaker, box, etc.) and must then superimpose the desired cancelling impedance onto the system. The characteristics of the electromechanical components have been thoroughly studied and described. As a result, the adaptation is possible only with complex transmission functions of the signal-shaping transducer. These transmission function can only be realized approximately.
One variant of the basic idea of hybrid sound attenuators are active Helmholtz resonators according to German patent document DE 42 26 855 and Spannheimer, H., Freymann, R., Fastf, H.: Aktiver Helmholtz-Resonator zur Daempfung von Hohlraumeigenschwingungen. Fortschritte der Akustik [Active Helmholtz resonator for attenuating self-induced cavity vibrations. Advances in Acoustics]xe2x80x94DAGA 1994, DPG-GmbH, Bad Honnef: 1994, pages 525 to 528 (compare FIG. 7, applied preferably in motor vehicles). In FIG. 7, a conventional Helmholtz resonator represents the passive sub-system described in German patent document DE 40 27 511, which is subjected to an active modification on its rear side.
In detail, the Helmholtz resonator, which is known per se, is defined by a hollow body 16 and an opening 17. A microphone 18, which is located outside the Helmholtz resonator (beside the opening), provides information about the prevailing acoustic pressure at the opening. Here, a transmission system 20 with specific (PDT) frequency and time response characteristics generates the required voltage for the loudspeaker 19 in the hollow body. This loudspeaker 19 determines or varies the transmission characteristics (resonance frequency) of the original Helmholtz resonator. Hence, the loudspeaker in the hollow body serves to practically enlarge (generally: change) the volume of the hollow body for an improved sound absorption of the Helmholtz resistor at low frequencies. Therefore, in this system, active reduction of the resonance frequency and thus the sound absorption of the passive Helmholtz resonator is sought.
It is an object of the present invention to improve the efficiency of the reactive sound attenuator consisting of a cavity with sound-proof limits and including at least one membrane, an acoustic sensor in the immediate vicinity in or on the membrane, as well as an electroacoustic transducer and an inverting signal amplifier, as well as to reduce the engineering expenditures involved with such an attenuator.
This and other objects and advantages are achieved by the reactive sound attenuator according to the invention, in which both the detection of, and an active modification of, the sound field occur directly and immediately on the duct wall.
In accordance with the objectives of the invention, the fundamental principle of the reactive sound attenuator, i.e., the exploitation or amplification of the membrane vibrations as sound field image directly in the duct wall provides various advantages over existing active sound attenuators.
In another objective of the invention, the reactive sound attenuator is operable without passive sub-systems (porous absorbers, Helmholtz resonators, etc.). This fact, as well as a spatial concentration of a membrane and a sensor in a duct wall, permit the use of a plain amplifier. Hence, all the components of the reactive sound attenuator can be integrated in a compact housing without any problems.
In another advantageous feature of the invention, several adjacent reactive sound attenuators are arranged in a three-dimensional cascade in the duct wall or in sound-reducing cells. This results in a correspondingly higher sound attenuation. The attenuation effect of reactive sounds attenuators in a cascade in the duct is practically limited only by secondary sound paths (in analogy with passive sound attenuators).
According to yet another object of the present invention, the reactive sound attenuator may be adapted to any sound fields and any sound field limits such as duct deflectors. The reactive sound attenuator cassettes and hence all electroacoustic components may be protected from physical and chemical loads occurring in the duct via acoustically pervious covers.
In an embodiment of the reactive sound attenuator, using a microphone as the sensor, the microphone is positioned behind the membrane, i.e., in the cavity of the cassette. The principle of operation of the reactive sound attenuator may not only be applied with plane waves in comparatively narrow ducts, but may be applied to achieve an attenuation of modal sound fields in any duct or space. In these applications, the vibrating membranes of the reactive cassettes equally ensure a reduction of the sound pressure on the area of the clad or lined wall surface, thus attenuating the sound field that exists there.